1. Technical Field
The present invention relates to antennas and particularly to cavity backed antennas.
2. Related Art
One type of antenna suitable, for example, for satellite communication is planar array antennas. Planar array antennas are generally formed of an array of many (e.g., hundreds) cells, defined at least in part on printed circuit boards.
In a simple antenna, each cell includes a single electric probe, which either receives electromagnetic signals from a remote antenna (e.g., a satellite carried antenna) or transmits electromagnetic signals toward a remote antenna. A bottom reflective layer of the planar antenna reflects electromagnetic signals propagating downward, such that they reflect upwards toward the remote antenna.
It has been suggested to use a dual beam and dual polarization antenna, in which each cell includes two orthogonal electric probes, in separate layers, such that the probes share a common cell aperture. In order to prevent interference between the probes in a single cell, intra-cell isolation is required.
U.S. Pat. No. 5,872,545 to Rammos, the disclosure of which is incorporated herein by reference, describes such a dual beam and dual polarization antenna. Intra-cell isolation between the beams, however, is limited in the Rammos antenna and therefore the antenna can not be used in applications which are sensitive to signal polarization.
The problem of isolation between the beams of a single cell is compounded in relatively large planar arrays, which are used for transmissions over a relatively large bandwidth (e.g., for communications). In such arrays, also inter-cell isolation is required between the cells. In order to prevent interference between the cells, for example, each cell may be surrounded by a metallic frame. While such metallic frames improve the radiation efficiency of each cell, they interfere with the intra-cell isolation and make it even harder to use dual-polarization cells.
U.S. patent publication 2003/0122724 to Shelley, et al., the disclosure of which is incorporated herein by reference, describes a planar array antenna with elements having two orthogonal probes. Features are described to increase isolation between the signals associated with each of the probes.